clandestine

Adjective

• conducted with or marked by hidden aims or methods
  • clandestine intelligence operations
    
  • cloak-and-dagger activities behind enemy lines
    
  • hole-and-corner intrigue
    
  • secret missions
    
  • a secret agent
    
  • secret sales of arms
    
  • surreptitious mobilization of troops
    
  • an undercover investigation
    
  • underground resistance
    

Related words

• Synonyms: 
  cloak-and-dagger , hole-and-corner(a) , hugger-mugger , hush-hush , secret , surreptitious , undercover , underground